One disadvantage that is characteristic of SAW filters is their frequency drift as a function of the temperature. This influence has a tendency to increase with the electro-acoustic coupling factor of the piezoelectric substrate that is used. For applications in which high selectivity between closely adjacent channels is important, for example, in the case of US PCS duplexers, conventional substrates such as LiTaO3 are not suitable for complying with the stringent requirements over a wide temperature range (for example, −40° C. to +85° C.). With a typical temperature coefficient of the mid-frequency (TCF, temperature coefficient of frequency) of ≈−42 ppm/K, this is, in this case, varied by more than 0.5%.
German patent document 10 2004 045 181 A1 discloses a SAW component with a reduced temperature response, in which an SiO2 layer is applied over the component structures in order to compensate for the temperature response, and the substrate is connected to a compensation layer on the lower face. The additional layers and processing steps increase the production variation of the product, and therefore, of course, also increase the costs.
U.S. Pat. No. 6,563,133 B1 discloses an SAW component in which a wafer bonding method is used for connection of the substrate to a compensation wafer. For this method, the surfaces of the substrate and of the compensation wafer to be connected must be extremely highly smooth and must be extremely free of particles. Furthermore, special ion implantation techniques and plasma activations are required.